1. Technical Field
The present invention generally relates to transmission control systems and, more particularly, to an interactive engine and automatic transmission control system for improving vehicle drivability during coast down gear shifts and accelerations.
2. Discussion
When an automotive vehicle changes speeds from a cruising rate to a stop or near stop condition, the transmission shifts from a high forward gear down to a low forward gear. This event is known in the art as a coast down shift. In terms of shift quality and driver expectation, the points where the shifts occur are critical.
Shift points are chosen so that the torque on the transmission either remains entirely positive or entirely negative before, during, and after the shift. Because of the wide range of engine speeds that can exist during a coast down event, the shift points are confined to very narrow ranges. Shifting anywhere outside of these ranges has the potential to cause a torque reversal of the drive train which is detectable to the driver as a bump or knock.
Also, when braking moderately from a high speed, a driver expects the deceleration rate of the vehicle to remain constant. However, if a down shift occurs at a relatively high speed, it is possible for the turbine speed to increase substantially above the engine speed. The difference in speed between the engine and turbine will produce a negative torque which will tend to decelerate the vehicle. This torque will add to the driver's braking and cause an objectionable change in the deceleration rate of the vehicle.
For the above reasons, coast down shift points have been traditionally confined to low speed ranges. However, in terms of vehicle performance, low shift speeds can be undesirable. For example, when turning a corner a vehicle should optimally be in a down shifted gear just after entering the corner so that a better engine response and acceleration is provided out of the corner when the driver steps back into the throttle. If a vehicle is in a high gear when exiting the corner, time must be taken to perform a down shift and the desired vehicle response is delayed.
In addition, after braking to a near stop, a driver may tip back into the throttle to accelerate. This often happens when braking for a stop light and then accelerating when the light turns green before a complete stop is achieved. By placing the transmission in the appropriate gear prior to the driver pressing the accelerator pedal, vehicle performance is improved. Unfortunately, to date no effective control system has been provided for adequately down shifting the transmission.
In view of the foregoing, it would be desirable to provide a transmission controller which interacts with the engine controller to increase engine speed above turbine speed during a coast down shift with spark and air flow control so as to improve drivability.